Multiline support for Toolkit Annotations

This is a follow-up issue started at https://github.com/actions/toolkit/issues/186#issuecomment-546772099.

It is unclear from the recently added Annotation docs if ::debug/::warning/::error messages support multiline text in the {message} placeholder.

Why it is important? These Annotations can be used by various code linters and other similar tools, but their output easily could be spanned to multiple lines, for example:

pylint output:

W:108: Operator not followed by a space
     print >>sys.stderr, 'Unable to match %r', line
            ^

Rust compiler output:

warning: comparison is useless due to type limits
  --> example/src/lib.rs:20:8
   |
20 |     if pid < 0 {
   |        ^^^^^^^
   |
   = note: `#[warn(unused_comparisons)]` on by default

It would be nice to clarify if it is possible to use multiline strings over there and it is expected to be working correctly in the future.

Read the linked comment on issue #186 and the Annotation docs first; the issue names no source file or test. Establish whether ::debug, ::warning, and ::error accept multiline {message} values, then document the behavior or define the required support and identify how it should be verified.